Electromagnetic interference shielding in an implantable medical device

ABSTRACT

EMI shields for use in implantable medical devices that include inner and outer metal layers separated by a dielectric layer. When assembled as medical devices, the outer metal layer of an illustrative EMI shield is placed into electrical contact with a conductive inner surface of an associated canister for an implantable medical device.

FIELD

The present invention relates to the field of implantable medicaldevices. More particularly, the present invention relates to implantablemedical devices that include internal shielding to preventelectromagnetic interference with circuitry contained in such devices.

BACKGROUND

Implantable cardiac stimulus devices, as well as many other implantablemedical devices, typically include control circuitry that is adapted toperform various functions such as sensing, communication and/or stimulusdelivery. Such devices operate within a patient's body, and are subjectto various sources of electromagnetic interference (EMI) including, forexample, noise from other electrical devices inside or outside of thepatient's body, power line noise, noise generated by the patient's bodyitself, and, for some devices, noise that the device itself generates.For example, implantable cardiac stimulus devices typically deliverelectric pulses to regulate or correct cardiac activity, and theirsensing algorithms are often configured to avoid capturingself-generated signals. Some such devices, known as implantablecardioverter defibrillators (ICDs), deliver very large stimuli to shocka patient's heart out of an arrhythmic state such as ventriculartachycardia or ventricular fibrillation. When large pulses aredelivered, it is desirable to limit the effects of the large pulse onoperation of internal circuitry. New and alternative designs forlimiting such effects in implantable medical devices are desired.

SUMMARY

The present invention, in an illustrative embodiment, includes animplantable medical device that includes operational circuitry containedin a housing. An EMI shield is disposed between the operationalcircuitry and the housing. The EMI shield, in an illustrativeembodiment, includes an inner conductive layer which is coupled to areference voltage. The EMI shield also includes an outer conductivelayer that is exposed on its outer surface to the interior of thehousing. The inner and outer conductive layers, which may be formed ofconductive metals, for example, silver or copper, are separated by adielectric layer. By exposing the outer conductive layer to contact withthe interior of the housing, air gaps between the outer conductive layerand the housing are prevented from becoming sources for nonlinearelectrical conduction such as corona discharge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show respective subcutaneous and transvenous cardiacstimulus systems;

FIGS. 2A-2B show perspective and cross-sectional views of an EMI shield;

FIG. 3 is an exploded view of an implantable medical device illustratingthe assembly of a canister, EMI shields, and operational circuitryincluding batteries and capacitors;

FIGS. 4A-4C illustrate, in plan and partial sectional views, anillustrative embodiment of an EMI shield;

FIG. 4D is a partial sectional view showing an alternative constructionto that shown in FIG. 4C;

FIG. 5 shows an oscilloscope output illustrating corona discharges whenthe design of FIGS. 2A-2B is used as an EMI shield during a simulatedhigh voltage signal application;

FIG. 6A illustrates, in perspective view, a PEEK insulating liner;

FIG. 6B shows an oscilloscope output illustrating corona discharges whenthe design of FIGS. 2A-2B is used with the insulating liner of FIG. 6Aas an EMI shield during a simulated high voltage signal application;

FIG. 7A illustrates, in perspective view, an EMI shield having varnishapplied along the edges thereof;

FIG. 7B shows an oscilloscope output illustrating corona discharges whenthe varnished EMI shield of FIG. 7A is used as an EMI shield during asimulated high voltage signal application;

FIG. 8A illustrates, in perspective view, a varnished canister;

FIG. 8B shows an oscilloscope output illustrating corona discharges whenthe design of FIGS. 2A-2B is used as an EMI shield inside the varnishedcanister of FIG. 8A during a simulated high voltage signal application;

FIG. 9 shows an oscilloscope output illustrating corona discharges whenthe design of FIGS. 2A-2B is used as an EMI shield while adhered to acanister during a simulated high voltage signal application;

FIG. 10 is a perspective view showing an illustrative embodimentincluding an EMI shield having metallized tape applied to the outsidethereof;

FIG. 11 illustrates, for comparison, a sectional view of the shield ofFIGS. 2A-2B in contact with a canister in contrast to a section view ofthe shield of FIG. 10 in contact with a canister;

FIG. 12 shows an oscilloscope output illustrating linear response whenthe EMI shield of FIG. 10 is used as a shield during a simulated highvoltage pulse;

FIGS. 13A-13B and 14A-14B show oscilloscope outputs comparingperformance of an EMI shield as in FIGS. 2A-2B to that of an EMI shieldas shown in FIG. 10 during delivery of simulated high voltage pulses;

FIGS. 15A-15B show oscilloscope outputs comparing performance of an EMIshield as shown in FIGS. 2A-2B to that of an EMI shield as shown inFIGS. 4A-4C; and

FIGS. 16A-16B are graphs showing expected versus average detectedcurrents for tested EMI shields.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

FIGS. 1A-1B, respectively, show subcutaneous and transvenous implantedcardiac stimulus systems relative to the heart. Referring to FIG. 1A,the patient's heart 10 is shown in relation to an implanted,subcutaneous cardiac stimulus system including a canister 12. A lead 14is secured to the canister and includes sensing electrode A 16, coilelectrode 18, and sensing electrode B 20. A can electrode 22 is shown onthe canister 12. Illustrative subcutaneous systems are shown in U.S.Pat. Nos. 6,647,292 and 6,721,597, and the disclosures of these patentsare incorporated herein by reference. Some embodiments include a unitarysystem having two or more electrodes on a housing as set forth in the'292 patent, rather than that which is shown in FIG. 1A. A unitarysystem including an additional lead may also be used.

Referring now to FIG. 1B, a transvenous system is shown relative to apatient's heart 30. The transvenous cardiac stimulus system includes acanister 32 connected to a lead 34. The lead 34 enters the patient'sheart and includes electrodes A 36 and B 38. Additional electrodes forsensing or stimulus delivery may also be included, and also may be usedfor sensing in some embodiments of the present invention. In theillustrative example, electrode A 36 is located generally in thepatient's ventricle, and electrode B 38 is located generally in thepatient's atrium. The lead 34 may be anchored into the patient'smyocardium. The lead 34 may also include one or more coil electrodes,either interior to or exterior to the heart, as shown at 42, which maybe used to deliver stimulus and/or to sense cardiac or other activity,such as respiration. A can electrode 40 is shown on the canister 32.With this system, plural sensing vectors may be defined as well, infirst and second polarities. In both FIGS. 1A and 1B, one or moresensing electrodes may also be used for stimulus delivery. Someembodiments of the present invention may be used in combination systemsthat may include sensing vectors defined between two subcutaneouselectrodes, a subcutaneous electrode and a transvenous electrode, or twotransvenous electrodes.

The systems shown in FIGS. 1A-1B may include operational circuitry and apower source housed within the respective canisters. The power sourcemay be, for example, a battery or bank of batteries. The operationalcircuitry may be configured to include such controllers,microcontrollers, logic devices, memory, and the like, as selected,needed, or desired for performing the illustrative methods set forthherein. The operational circuitry may (although not necessarily) furtherinclude a charging sub-circuit and a power storage sub-circuit (forexample, a block of capacitors) for building up a stored voltage forcardiac stimulus taking the form of cardioversion and/or defibrillationpulses or stimuli. The operational circuitry may also be adapted toprovide a pacing output. Both cardioversion/defibrillation and pacingsub-circuitry and capacities may be incorporated into a single device.Methods of signal analysis may be embodied in hardware within theoperational circuitry and/or as instruction sets for operating theoperational circuitry and/or in the form of machine-readable media(optical, electrical, magnetic, etc.) embodying such instructions andinstruction sets.

In illustrative examples, a cardioversion/defibrillation pulse may besupplied by a transvenous ICD in a variety of amplitudes, energy levels,and formats. Biphasic and monophasic waveforms can be used. Constantvoltage or constant current formats may be used, though it is typical toprovide an output that is “tilted,” that is, output voltage decays froman initial value over time as the energy storage circuit of the ICDdischarges. Tilt is measured in terms of final voltage relative toinitial voltage. For example, an existing line of Medtronic® transvenousdevices (GEM® II VR) can be programmed to deliver initial outputvoltages of 83-736 volts with 0.4 to 30 Joules of delivered energy in abiphasic waveform with 50% tilt (assuming delivery into 75 ohms ofresistance). Depending upon electrode placement and energy delivery,voltages as low as 50 volts may be useful in some ICDs. SubcutaneousICDs are being developed and are expected to utilize voltage outputsthat will include at least the upper portions of the delivery energy andvoltage ranges for transvenous devices, while also using higher deliveryenergies and voltages when necessary. For example, delivery voltages inthe range of 1350 volts, with energy in the range of 30-40 Joules, andup to 80 Joules, or more, are expected to be within the range of suchdevices, although higher and lower values may be realized. Electrodepositioning can play a role in modifying such ranges. These values aremerely illustrative and should not be taken as limiting.

Each of the devices 12, 32 may further include such components as wouldbe appropriate for communication (such as RF communication or inductivetelemetry) with an external device such as a programmer. To this end,programmers 24 (FIG. 1A) and 42 (FIG. 1B) are also shown. For example,during an implantation procedure, once the implantable device 12, 32 andleads (if included) are placed, the programmer 24, 42 may be used toactivate and/or direct and/or observe diagnostic or operational tests.After implantation, the programmer 24, 42 may be used to non-invasivelydetermine the status and history of the implanted device. The programmer24, 42 and the implanted device 12, 32 are adapted for wirelesscommunication allowing interrogation of the implanted device. Theprogrammers 24, 42 in combination with the implanted devices 12, 32 mayalso allow annunciation of statistics, errors, history and potentialproblem(s) to the user/physician. The particulars of operationalcircuitry, signal analysis, lead placement, implantation, communicationand programmers may vary widely in embodiments associated with thepresent invention.

FIGS. 2A-2B show a perspective and a cross-sectional view of an EMIshield. The shield 60 includes a solder pad 62 that allows soldering ofa layer of the EMI shield to the ground plane of the associatedcircuitry. During assembly, a relatively small patch-type pad may beplaced over the solder pad 62 to electrically isolate it from anassociated canister.

As shown by FIG. 2B, a cross section of the EMI shield shows an outerdielectric layer 64, which covers a metal layer 66, which is placed onan inner dielectric layer 68. In an illustrative example, the dielectriclayers 64, 68 include 1 mil of polyimide. At the edges of the shield,the metallic layer 66 may be pulled back to reduce edge effects. Anyconductive metal or alloy maybe used as metallic layer 66; inillustrative examples, copper and/or silver are used. In an illustrativeexample, the metallic layer 66 was pulled back 10 mils from the edge ofthe EMI shield 60. Further, in the illustrative example, the solder pad62 was used to tie the metallic layer 66 to a reference voltage (i.e.,ground) for the overall device. Certain shortcomings of this design areexplained in further detail below. The EMI shield 60 is used by placingit between housed operational circuitry and a canister provided to housethe operational circuitry, as shown by FIG. 3.

FIG. 3 is an exploded view of an implantable medical device illustratingthe assembly of a canister, EMI shields, and operational circuitryincluding batteries and capacitors. The canister includes a firstcomponent 80 and a second component 82. The first and second components80, 82 may be made of any suitable biocompatible material. Titanium isan illustrative material, although other materials may be used in placeof or in combination with titanium. Portions of the outside of the firstand second components 80, 82 may be coated, shaped, or treated in anysuitable fashion. In some embodiments, the first and second componentsmay be configured to matingly fit together, for example, in a snap fitor an overlapping fit. Typically, the completed device will have a weldseam joining the first component 80 to the second component 82, althoughadditional intermediate members may also be included on the inside oroutside of the device, and welding need not be used for some embodimentsusing, for example, adhesive or snap-fit.

Internal parts shown in the exploded view include a first EMI shieldportion 84 and a second EMI shield portion 86. A solder pad is shown onthe first EMI shield portion 84. Sandwiched between the EMI shieldportions 84, 86 is the operational circuitry of the device. In theillustrative embodiment shown, the operational circuitry is shown in ahighly simplified fashion, and includes a capacitor block 88, controlcomponents 90, and a battery 92. The operational circuitry shown islikely for such devices as ICDs or other devices that provide electricalstimuli to a patient. The precise details of the control componentsand/or the operational circuitry generally may vary widely dependingupon the desired functionality of the device.

Generally, the operational circuitry will define a ground potential foroperation of its circuitry. A reference output, which may be theoperational circuitry ground or some other voltage defined relative tothe operational circuitry ground, may be electrically connected to themetal layer of an associated EMI shield at the solder pad. A frame (notshown) may be included to hold the operational circuitry parts 88, 90,92, in place.

While much of the present description is directed toward implantablecardiac stimulus devices, particularly ICDs, it should be understoodthat the concepts, devices and methods disclosed herein for providingEMI shielding in an implantable medical device can be applied morebroadly in the field of implantable medical devices. This may includeother implantable devices that house electronics and are susceptible tonoise interference.

A number of the Figures that follow show oscilloscope outputs that weregenerated during actual testing of devices during simulated high voltagepulse delivery. The testing methods can be understood by viewing theexploded view of FIG. 3. The illustrative tests referred to in theFigures which follow were performed by providing one of the EMI shieldportions 84, 86 against a corresponding canister component 80, 82.Substitutes for the relatively expensive operational circuitrycomponents that would be used in an actual device were provided,including a non-functional battery, capacitors and an associated framethat would be used in an actual device to hold the operational circuitrytogether in place within the canister. Weights were placed on these“substitutes” to hold everything in place, but the second side of thecanister was not attached, such that the internal components,particularly the EMI shield portion 84, 86 remained accessible. Intesting, a voltage was applied between a sandwiched metal layer of theshield portion 84, 86 and the metallic canister component 80, 82.Resultant currents were then observed. This simulates application of astimulus pulse by the use of an electrode disposed on the canister incombination with an electrode disposed on a lead. These methods wereused in generating the following figures, with the exception of FIGS.15A-15B and 16A-16B, which provide information captured using differenttesting conditions.

For FIGS. 6B, 7B, 8B, 9, 12, 13A-13B and 14A-14B, testing was performedusing a 60-Hz output. The oscilloscope views in these Figures werecaptured with an applied signal of 1000 Vrms. Nonlinearities caused bycorona discharge show up as spikes on the oscilloscope outputs. Actualmeasurement of the amount of current caused by the corona discharge wascalculated by monitoring the voltage across a series 10 kilohm resistor.This form of simulation of high voltage pulse delivery is believed toprovide a reasonable and useful understanding of whether and how wellthe proposed EMI shields performed with respect to corona discharge.

FIGS. 4A-4C show, in plan and partial sectional views, an illustrativeembodiment of an EMI shield. The EMI shield is shown generally at 100,and is designed to have first and second components connected by anarrow bridge member, allowing it to fold around operational circuitry.The EMI shield 100 may be fabricated in any manner allowing for themulti-layered constructions described herein. For example, the EMIshield 100 may be manufactured as a flexible printed circuit board. Inthe embodiment shown, the canister for the implantable medical deviceincludes first and second major faces, with the EMI shield 100 shaped asshown to correspond to the major face(s) of the device. In otherembodiments, the EMI shield 100 may be shaped as desired. For example,conformal ICDs are shown in U.S. Pat. No. 6,647,292, having longer,curved housings; an EMI shield 100 may be shaped or formed differentlyfor such applications. The EMI shield 100 may also be sized to coveronly a desired region of the implantable medical device.

FIG. 4B highlights details around a solder pad 120 in the EMI shield 100in FIG. 4A. The details of the illustrative EMI shield 100 that areshown in FIG. 4B away from the solder pad 120 may be consistent with therest of the EMI shield 100 except for its edges. A first dielectriclayer 102 has an outer metal layer 104 thereon. In an illustrativeembodiment, the first dielectric is polyimide, though other dielectricmaterials may be used. An inner metal layer 106 is secured to the firstdielectric layer 102. The exact construction may vary, for example,depending upon the manner of fabrication used. For example, in someembodiments, the EMI shield 100 may be constructed of separate layersthat are assembled together using adhesives; in other embodiments, theEMI shield 100 may be formed by deposition processes. In theillustrative example that is shown, the metal layers 104, 106 areformed/placed on the first dielectric layer 102 in a process forming aflexible printed circuit board. If desired, the entire device may bemade in such a manner, including the additional second dielectric layer110.

In the illustrative EMI shield 100, a second dielectric layer 110 isalso provided inside of the inner metal layer 106 to isolate housedoperational circuitry from undesired or inadvertent contact with theinner metal layer, which may be coupled to a reference output or groundof the housed operational circuitry. While the second dielectric layer110 may be omitted in some embodiments, it will often serve to reduce orlimit cross talk and/or inadvertent shorting of sub-circuits in thedevice by covering some, a majority, or nearly all of the inner metallayer 106. In an illustrative embodiment, the second dielectric layer110 is ESPANEX™ SPC-35A-25A, a laminate-ready commercially availablepolyimide coverlay with an adhesive 108 already provided thereon,allowing it to bond to the inner metal layer 106. Other dielectricmaterials may be used. The metal layers 104, 106 may be formed of anysuitable conductive metal, such as silver, copper, etc., and may beselected in view of various factors such as durability, cost, resistanceto corrosion, ease of manufacture, bonding or handling, andbiocompatibility, for example.

FIG. 4B also shows that at the solder pad 120, the outer metal layer 104may be pulled back such that it is separated from the portion 112 of theinner metal layer 106 that is provided to allow secure soldering. Asuitable connection, such as a conductive wire, can be soldered from theoperational circuitry to the solder pad 120, allowing the inner metallayer 106 to be grounded. The exposed portion 112 of the inner metallayer that extends across the first dielectric layer 102 can be covered,after soldering, with an additional dielectric patch before placing acanister thereover.

FIG. 4C illustrates a perimeter portion of the EMI shield 100. In theillustrative embodiment, the outer metal layer 104 extends virtually tothe edge of the perimeter portion, while the inner metal layer 106 endsa distance away from the edge, defining a pull-back region along theperimeter. In illustrative embodiments, the pull-back region may have awidth of from about 1 mil to about 100 mils, for example. By pulling theinner metal layer 106 back from the edge, the likelihood ofnonlinearities (such as corona discharges) is reduced, at least at theedge of the EMI shield.

The dielectric layers 102, 110 may have thicknesses in the range ofabout 1-10 mils, although this may vary. In an illustrative embodimentfurther discussed below, the dielectric layers 102, 110 are about 2 milsthick, and the inner metal layer 106 is pulled back about 60 mils fromthe edge of the EMI shield 100.

FIG. 4D is a partial sectional view showing an alternative constructionto that shown in FIG. 4C. In the alternative construction, an EMI shield130 includes an outer metal layer 132 having a portion that extendsaround the edge of the shield, as shown at 134. Again, the inner metallayer 138 is shown ending a distance away from the edge of the perimeterof the EMI shield 130. An adhesive 144 may be used to secure the innermetal layer 138 to a second dielectric layer 142, as well as to join thefirst dielectric layer 136 and second dielectric layer 142 in thepull-back region 140 between the perimeter of the EMI shield 130 and theouter perimeter of the inner metal layer 138 and the edge of theperimeter of the EMI shield 130. The dielectric layers 136, 142 may havediffering thicknesses, as shown.

Referring briefly back to FIG. 3, it can be seen that the edge of theEMI shield may be exposed to the interior of the canister. In theembodiment of FIG. 4D, extending the outer metal layer 132 to wraparound the edge of the perimeter of the EMI shield 130, as shown at 134,provides an additional “touch-point” for touching the outer metal layer132 to the canister (see FIG. 3). Further, the “air gap”, which isfurther explained below, can be eliminated along this portion of thedevice. As further illustrated in FIG. 11, the provision of one or moretouch points between the conductive outer metal layer 132 and thecanister may aid in reducing corona discharge.

FIG. 5 shows an oscilloscope output illustrating corona discharge whenthe EMI shield of FIGS. 2A-2B is used as a shield during a simulatedhigh voltage pulse. As explained above, the testing methods used applieda 60-Hz sinusoidal signal. A problem with the waveform in FIG. 5 is thenonlinearities that are visible at 190, 192. These spikes 190, 192 arecaused by corona discharges that occur across the air gaps between theouter dielectric layer 64 (FIG. 2B) of the EMI shield and the interiorof the canister. These corona discharges can become large enough to bevisualized as sparks along the outside edge of the EMI shield under theright circumstances.

The corona discharge may be the cause of at least some system resets, aswell as other electronics problems that occur during testing of ICDsusing the shield shown in FIGS. 2A-2B. To provide a rough measure of thefrequency and amplitude of such spikes, the above described testingsetup and procedure was used. Prior to applying a signal, thecapacitance of the testing structure was determined using a commerciallyavailable device for testing capacitance. Using a formula relating RMScurrent to frequency, voltage and capacitance (I=2πf*C*V), an expectedcurrent was determined. Actual current was then monitored duringtesting. Comparing the actual current to the expected current providesan estimate of the effectiveness of the EMI shield in preventing coronadischarges.

The results for the EMI shield of FIGS. 2A-2B showed individual coronadischarges of up to 1.5 mA, and a difference between average andexpected RMS current of about 0.6 mA rms at 1000 Vrms, meaning that theaverage current about tripled the expected current. The oscilloscopeoutput shown in FIG. 5 clearly shows large spikes resulting from coronadischarges occurring at and near the signal peaks. In testing,nonlinearities could be detected at voltages as low as 300 Vrms.

FIG. 6A illustrates, in a perspective view, a PEEK insulating liner 200.The PEEK liner 200 is about 4 mils thick, and is shaped to be placedbetween an EMI shield as shown in FIGS. 2A-2B and a canister for animplantable medical device. FIG. 6B shows the oscilloscope output forthe instantaneous current using the PEEK liner 200 in addition to an EMIshield as in FIGS. 2A-2B. The scale is the same in FIG. 6B as in FIG. 5.The average current was greatly reduced by the addition of moreinsulation. However, current spikes from corona discharge are alsoclearly visible. As measured at 1000 Vrms applied signal, the differencebetween average and expected current is in the range of 0.023 mA rms,and corona discharges of up to 0.5 mA were identified. The increase inaverage current was in the range of 20% relative to expected current.

Additional modifications to the original shield were tried as well.These included doubling the thickness of the polyimide dielectric layersto 2 mils, and pulling the metallic layer back 60 mils from the edge,rather than the original 10 mils. These tests showed a difference of0.095 mA rms between average and expected current at 1000 Vrms, nearlydoubling the current, and individual corona discharges as large as 0.5mA. Extra insulation on the face and edges was an improvement, butcorona was still prevalent.

FIG. 7A illustrates, in perspective view, an EMI shield 210 havingvarnish 212 applied to the outer edges thereof, and varnish 216 appliedaround solder pad 214. The applied varnish 212, 216 was an insulatingvarnish with an insulating strength in the range of 1000 V/mil. As shownin the oscilloscope output of FIG. 7B, a strong out-of-phase currentresulted at 1000 Vrms, with relatively large and frequent coronadischarge for the EMI shield of FIG. 7A. A difference of 0.82 mA rmsbetween expected and average current resulted, nearly tripling thecurrent, with spikes as large as 0.7 mA.

FIG. 8A illustrates, in perspective view, a varnished canister. Thevarnish 222 was applied to the entire interior of the can 220. Again,the applied varnish was an insulating varnish with an insulatingstrength in the range of about 1000 V/mil. As shown in FIG. 8B, thevarnished canister again provided a strong out-of-phase component, withcorona discharge still occurring, although with less amplitude andfrequency. In testing, at 1000 Vrms applied signal, the differencebetween average and expected current was about 0.39 mA rms, nearlytripling the expected current, with spikes as large as 0.3 mA. Fullinsulation on the can reduced corona, but did not eliminate it.

FIG. 9 shows an oscilloscope output illustrating corona discharge whenthe EMI shield of FIGS. 2A-2B is used as a shield while adhered to acanister during a simulated high voltage pulse. Here, adhesive wasapplied to the interior of a canister, and the EMI shield was placedtherein, with the aim of reducing and/or eliminating air gaps acrosswhich corona discharge formed in the above tests. At 1000 Vrms, thedifference between expected and average current was about 0.186 mA rms,representing a change of around 20%, with individual discharge spikes aslarge as 0.4 mA. Corona discharges were still present with adhesive, butthey were greatly reduced simply by bonding the shield to the can. Sincethis adhesive only covered approximately 75% of the shield surface area,it was not fully effective.

FIG. 10 is a perspective view showing an illustrative embodiment of thepresent invention including an EMI shield 240 having metal tape 242applied to the outside thereof. The aim was to eliminate air gaps havinglarge voltages across them. The metallized tape 242 would conductelectricity from the can to itself, eliminating voltage across air gapsbetween the EMI shield 240 and the outer can. Because it was adheredfirst to the EMI shield 240, the metallized tape 242 would not introduceadditional air gaps between its metal and the metal layer of the EMIshield 240, placing the voltage across only the dielectric. Thedielectric would now include the polyimide layer and any adhesivesbetween the EMI shield 240 metal layer and the metal layer on the metaltape 242.

FIG. 11 provides an exaggerated illustration comparing a sectional viewof a shield 250 as in FIGS. 2A-2B in contact with a canister 252 to asectional view of a shield 260 as in FIG. 10 in contact with a canister266. At 254, an air gap is seen between the shield 250 (which includesexaggerated curvature) and the canister 252. Supposing an applied1400-volt pulse, the potential across the air gap would be about 1400volts, possibly enough to induce breakdown such as corona discharge,depending upon humidity, temperature, and the size of the gap. Thesurface of the EMI shield 250 formed by the dielectric will have avoltage gradient due to its high resistivity. The contact between theEMI shield 250 and the canister 252 does not eliminate the voltageacross air gaps.

The other EMI shield 260 includes an inner metal layer 262, a dielectric263, and an outer metal layer 264. As shown at 268, air gaps may alsooccur with the EMI shield 260. However, the conductivity of the outermetal layer 264 eliminates the voltage potential across the air gap. Thevoltage gradient across the surface of the metal layer will be minimalcompared to that of the dielectric surface on the other EMI shield 250.The “touch points” that surround the air gap at 268 short the voltageacross the air gap, preventing corona discharge.

FIG. 12 shows an oscilloscope output illustrating linear response whenthe EMI shield of FIG. 10 is used with high voltage applied. The resultsin FIG. 12 show substantial elimination of the corona discharge. Thedifference between average and expected current at 1000 Vrms was about0.07 mA rms. Current spikes of individual discharges were not detectableon the same scale as the other designs; changing the scale of theoscilloscope showed infrequent current spikes of less than 0.06 mA. Thisprototype EMI shield used metallized tape, and was rather rough in itsexecution (i.e., there may have been gaps between tape pieces, flaws inthe insulation due to handling, and the tape may not have bondedperfectly, leaving internal air gaps, etc.). It was expected thatfurther refinement, for example, construction of the EMI shield as shownin FIGS. 4A-4C, would improve performance.

Another prototype having the metallized tape was prepared, this timeusing an EMI shield having double the insulation (2 mils of polyimideinstead of 1 mil) and including a metal layer pulled back 60 mils,rather than 10 mils, from the edges. This improved on the performance,and reduced the difference between average and expected current at 1000Vrms to 0.016 mA rms. Current spikes were again infrequent, and thistime had amplitudes of less than 0.03 mA. Compared to the originallytested shield of FIGS. 2A-2B, the frequency and amplitude of coronadischarges was greatly reduced. At 1000 Vrms, the average current wasreduced from 0.6 mA rms to 0.016 mA rms ( 1/38^(th)) and maximum coronaamplitudes reduced from 1.5 mA to 0.03 mA ( 1/50^(th)).

FIGS. 13A-13B and 14A-14B show oscilloscope outputs comparing theperformance of a shield as in FIGS. 2A-2B to that of a shield havingdoubled insulation (2 mils of polyimide), with an inner metal layerpulled back 60 mils (as compared to 10 mils), and including metallizedtape on the outside. This time, high voltage pulses were tested. FIG.13A shows the oscilloscope for a 1350-volt shock waveform using the EMIshield of FIGS. 2A-2B. Large spikes are clearly shown at 300, 302, andeven at 304, with a peak amplitude of the corona discharges being in therange of 80 mA. Meanwhile, as shown in FIG. 13B, which uses the samescale as FIG. 13A, no corona discharge spikes are seen with the EMIshield having doubled insulation, a larger pull-back region, and metaltape. A broader scale is shown in FIGS. 14A-14B, further highlightingthe differences in performance.

Further prototypes were prepared, this time in accordance with thedesigns of FIGS. 4A-4C. Six EMI shields (three of each of the two types)were tested. The testing involved using an external power supply for thesystem, but the internal control circuitry for an implantablecardioverter defibrillator was powered and active during shock delivery,in order to observe whether the control system reset during the shockdelivery. Telemetry was also performed to assess the effect of the EMIshields on the rate of framing errors that occurred during telemetrycommunications.

During delivery of shock waveforms near 1380 Volts, control circuitry indevices having shields similar to those shown in FIGS. 2A-2B resetduring shock delivery 62/80, 13/53, and 14/24 times for the threedifferent prepared shields. FIG. 15A shows the oscilloscope output forone of the shocks delivered with the shield of FIGS. 2A-2B, and includessignificant apparent corona discharge effects. In contrast, controlcircuitry in devices having the shields as shown in FIGS. 4A-4C, usingamplitudes in the same range of 1380 volts, did not reset a single timeduring 231 tests (0/80, 0/80 and 0/71 for the three prepared EMIshields). Testing used the same three sets of circuitry for both seriesof tests, in order to show that the shields themselves, rather than thecircuitry, caused the difference in performance.

FIG. 15B shows the oscilloscope output for one of the shocks deliveredwith the shield of FIGS. 4A-4C in place, and does not include the coronadischarge effects seen with the other shield. During testing, there wasone device which failed. However, this was determined to be caused by anerror during assembly that caused damage to a system component, and wasnot related to the efficacy of the EMI shield. It was found that thesets of shields performed comparably with respect to framing errors andnoise.

With respect to measurement of expected and average current, the metaltape prototype testing was further confirmed. For the devices having theEMI shields as shown in FIGS. 2A-2B, corona discharge was apparent inresponse to applied 60-Hz sinusoid at 1000 Vrms and 2000 Vrms, withspikes as large as 2 mA, and with corona discharge appearing at appliedvoltages exceeding 240 Vrms. Spiking was not detected for the EMIshields as shown in FIGS. 4A-4C in response to an applied 60-Hz sinusoidat 1000 Vrms, with testing including observation at scales that wouldshow spikes as small as 0.01 mA. At 2000 Vrms, the EMI shields as shownin FIGS. 4A-4C allowed current spikes in the range of 0.03 mA inamplitude, with these relatively small current spikes being firstobserved at around 1050 Vrms.

FIG. 16A shows results for expected versus average current with the EMIshields as shown in FIGS. 2A-2B for three tested EMI shields.Significant deviation from the expected current occurred for these EMIshields. FIG. 16B shows results for EMI shields as shown in FIGS. 4A-4Con the same scale used in FIG. 16A. In contrast to the other EMIshields, minimal deviation occurs, indicating very limited coronadischarge.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departures in form anddetail may be made without departing from the scope and spirit of thepresent invention as described in the appended claims.

1. An implantable cardiac stimulus device (ICSD) comprising: operationalcircuitry adapted to sense cardiac activity and provide cardiac stimulusoutput; a canister formed of a conductive metal and shaped for housingthe operational circuitry, the housing having first and second majorfaces; and an EMI shield comprising an outer metal layer and an innermetal layer with a dielectric therebetween; wherein the EMI shieldseparates the operational circuitry from one of the major faces of thehousing such that the outer metal layer is in electrically conductivecontact with the canister.
 2. The ICSD of claim 1, wherein the outermetal layer is electrically isolated from the inner metal layer of theEMI shield.
 3. The ICSD of claim 1, wherein the inner metal layer of theEMI shield is electrically connected to a reference output of theoperational circuitry.
 4. The ICSD of claim 1, wherein: the EMI shieldincludes an outer perimeter; the inner metal layer covers a majorportion of a first side of the EMI shield relative to the outerperimeter; and the inner metal layer does not extend to a pull-backregion adjacent the outer perimeter.
 5. The ICSD of claim 4, wherein thepull-back region has a width of about 60 mils.
 6. The ICSD of claim 4,wherein the outer metal layer extends around the dielectric at theperimeter.
 7. The ICSD of claim 1, wherein the EMI shield furtherincludes an inner dielectric layer covering a major portion of the innermetal layer and substantially isolating the operational circuitry fromcontact with the inner metal layer.
 8. The ICSD of claim 1, wherein: afirst electrode is disposed on the canister; the ICSD further comprisesa lead assembly coupled to the operational circuitry via a headerprovided on the canister, the lead assembly including at least a secondelectrode; and the operational circuitry includes high-voltagecapacitors and a battery system and is adapted to deliver a stimulusoutput in the range of more than 50 V in amplitude using an electrodesystem including the first and second electrodes.
 9. The ICSD of claim1, wherein a first electrode is disposed on the canister; the ICSDfurther comprises a lead assembly coupled to the operational circuitryvia a header provided on the canister, the lead assembly including atleast a second electrode; and the operational circuitry includeshigh-voltage capacitors and a battery system and is adapted to deliver astimulus output sufficient to achieve cardioversion/defibrillation of apatient into whom the ICSD can be implanted.
 10. An implantable medicaldevice (IMD) comprising: operational circuitry adapted to sense cardiacactivity; a canister formed of a conductive metal and shaped for housingthe operational circuitry; an EMI shield comprising an outer metal layerand an inner metal layer with a dielectric therebetween; wherein the EMIshield is disposed between the operational circuitry and at least aportion of the canister such that the outer metal layer is inelectrically conductive contact with the canister.
 11. The IMD of claim10, wherein the outer metal layer is electrically isolated from theinner metal layer of the EMI shield.
 12. The IMD of claim 10, whereinthe inner metal layer of the EMI shield is electrically connected to areference node of the operational circuitry.
 13. The IMD of claim 10,wherein: the EMI shield includes an outer perimeter; the inner metallayer covers a major portion of a first side of the EMI shield relativeto the outer perimeter; and the inner metal layer does not extend to apull-back region adjacent the outer perimeter.
 14. The IMD of claim 13,wherein the pull-back region has a width of about 60 mils.
 15. The IMDof claim 13, wherein the outer metal layer extends around the dielectricat the perimeter.
 16. The IMD of claim 10, wherein the EMI shieldfurther includes an inner dielectric layer covering a majority of theinner metal layer and substantially isolating the operational circuitryfrom contact with the inner metal layer.
 17. An EMI shield for use in animplantable medical device (IMD), the EMI shield comprising: an outermetal layer; an inner metal layer; and a first dielectric separating theouter metal layer from the inner metal layer; wherein the EMI shield issized and shaped for placement between operational circuitry of the IMDand a housing for the IMD.
 18. The EMI shield of claim 17, furthercomprising a second dielectric disposed over a major portion of theinner metal layer, the inner metal layer including a solder pad forattaching a wire thereto.
 19. The EMI shield of claim 17, wherein theEMI shield includes a perimeter portion and the inner metal layer doesnot extend to the perimeter portion.
 20. The EMI shield of claim 19,wherein the outer metal layer wraps around the perimeter portion.
 21. Amethod of manufacturing an implantable medical device comprising:providing housing elements including at least first housing elementhaving an inner face and an outer face, the first housing element havinga conductive metal surface on at least the inner face; providingoperational circuitry adapted to perform methods of the implantablemedical device; providing an EMI shield including an outer metal layer,an inner metal layer, and a first dielectric layer therebetween; andassembling the housing elements and the operational circuitry such thatthe EMI shield separates the operational circuitry from the firsthousing element and outer metal layer is in electrical contact with theconductive metal surface on the inner face.
 22. The method of claim 21:wherein the operational circuitry has a reference voltage output; andthe method further comprises electrically coupling the inner metal layerto the reference voltage output of the operational circuitry.
 23. Amethod of manufacturing an implantable cardiac stimulus device (ICSD)comprising: providing first and second canister sections formed of aconductive metal, each of the first and second canister sectionsincluding a major face and sides; providing an EMI shield havingcomponents corresponding to each of the first and second canistersections, the EMI shield having an outer metal layer, an inner metallayer, and a first dielectric layer therebetween; providing operationalcircuitry for the ICSD including a battery, a capacitor block, andcontrol circuitry; and assembling the first and second canistersections, the EMI shield, and the operational circuitry such that theouter metal layers of the EMI shield components are electrically coupledto corresponding interior surfaces of the first and second canistersections, with the EMI shield components placed between the operationalcircuitry and respective first and second canister sections.
 24. Themethod of claim 23: wherein the operational circuitry includes areference voltage output; and the method further comprises electricallycoupling the inner metal layers of each of the EMI shield components tothe reference voltage output.
 25. The method of claim 23, wherein theEMI shield components each have an outer perimeter and the inner metallayer does not extend to a perimeter section having a width of at least10 mils.
 26. The method of claim 25, wherein the EMI shield componentseach have outer metal layers that extend to and wrap around the outerperimeters.
 27. An implantable cardioverter-defibrillator (ICD)comprising: operational circuitry including a battery, energy storagecircuitry, and control circuitry, the operational circuitry configuredto monitor cardiac activity of a patient by receiving signals fromimplanted electrodes and to deliver stimulus to the heart of the patientin response to an identified malignant arrhythmia; a canister includingat least some regions of conductive material; and an electromagneticinterference (EMI) shield disposed between the operational circuitry andthe canister, the EMI shield comprising an outer metal layer and aninner metal layer with a dielectric therebetween, the EMI shieldconfigured to substantially prevent nonlinear effects during delivery ofstimulus.
 28. The ICD of claim 27 wherein the EMI shield is configuredto limit the occurrence of voltage differentials across air gaps betweenthe EMI shield and the canister.